1. Field of the Invention
The present invention pertains to a vehicle, such as a motorcycle, or even a bicycle, having a frame with a pivotally attached steering assembly, sometimes termed a fork assembly, as may be seen in FIG. 1. The steering assembly 2 typically consists of two parallel telescoping fork tubes 10, rigidly attached near their top ends to two triple trees 30, or clamping plate assembly. The steering assembly also consists of a front wheel 3 rotatably mounted on a front axle. The front axle is located transversely between the two fork tubes 10, near their bottom ends. Handlebars, a front fender, a braking mechanism, dampers, headlight, and a locking mechanism may be part of the steering assembly 2.
A frame head tube 6 is a tubular structural member at the vehicle's frame front portion 14. The frame head tube 6 is typically rigidly attached, by welding, to a frame top tube 25 and a frame down tube 26. Frame head tube 6 has a throughbore having a head tube axis 7. The head tube axis 7 is coaxial with, and thus defines, a steering axis 8. The throughbore contains bearings which receive a fastener such as threaded pin 29. Pin 29 is coaxial with the frame head tube axis 7 and the steering axis 8. Pin 29 also passes through the triple tree 30 component of the steering assembly. The steering assembly 2 pivots on the pin 29 about the steering axis 8, thus providing a steering function, so that an operator may steer the vehicle.
The steering axis 8 is a straight line coplanar with a vertical longitudinal plane, and is also inclined rearward with respect to a vertical line 22. The angle between the steering axis 8 and a vertical line 22 is called a steering inclination angle 23. The steering inclination angle 23 is arranged such that the steering axis 8 intersects the ground in front of the point at which the front tire contacts the ground. The horizontal dimension between these two points is called trail, and results in steering stability. The wheelbase of the vehicle is the horizontal distance between the point at which the front tire contacts the ground, and the point at which the rear tire, mounted on a rear wheel 4, contacts the ground.
Instead of telescoping fork tubes 10, the steering assembly 2 may include various, known in the art, springer or girder types of articulated linkage mechanisms, with springs and dampers, to support the front wheel 3.
2. Description of the Prior Art
Many motorcycle operators wish to relocate the steering assembly, in order to alter the steering geometry by changing the steering inclination angle 23. Many operators also wish to lengthen the wheelbase, to lower the motorcycle, to use longer fork tubes, in order to alter the function, or the appearance, of the motorcycle.
The most common method for relocating the steering assembly and steering axis, of a vehicle such as a motorcycle, is to first disassemble the steering assembly from its pivoting attachment at the motorcycle's frame head tube. Then the motorcycle's frame is completely severed, or chopped off, just rearward of the frame head tube, such that the frame head tube is completely detached from the remaining portion of the frame. Then, the frame head tube can be relocated, by rotation and or translation, in the vertical longitudinal plane, to a desired new position and angle, and then welded back onto the remaining portion of the frame, using small wedges of metal to form strengthening gussets. The steering assembly, or forks, can then be reinstalled onto the relocated frame head tube. This well established method, the typical results of which are illustrated in FIG. 2, is known as chopping a motorcycle. Chopping has several disadvantages, including the need for major disassembly of at least the front portion of the motorcycle, because the welding step can adversely affect, by melting, nearby wiring and electrical components. The cutting and welding can be a fire hazard, and may also damage any nearby painted or chromed surfaces. Precision alignment jigs and fixtures are preferably used to ensure proper alignment of the steering axis so that it remains within the vertical longitudinal plane. The resulting modification to the motorcycle's frame is permanent. Any error in alignment of the steering axis will result in misalignment of the steering, possibly causing unwanted erratic handling characteristics. Such alignment errors may include not maintaining the steering axis in a vertical longitudinal plane. After the cutting and welding has been accomplished, reassembly of every component which needed to be removed is then required, as is repainting of at least the front of the frame and head tube. If the welds are inadequate, catastrophic frame structural failure, during subsequent use of the motorcycle, is known. This chopping method does have the very desirable benefit of providing a relocated fork assembly having any desired steering inclination angle. The resulting chopper style motorcycle, usually having a steering axis more horizontally inclined than that of an unmodified motorcycle, permits use of longer fork tubes. Choppers are, however, very distinctive in appearance and can be very desirable, and may have very stable handling characteristics due to an increased trail dimension. However, the cost, effort, and specialized fabrication and measurement tools required to perform this modification discourages many motorcyclists from doing so. The traditional cutting and welding of the motorcycle's frame also discourages many owners of antique, brand new, or otherwise valuable motorcycles, from performing this irreversible modification.
U.S. Pat. No. 3,866,946 provides a mechanism for the adjustment of the steering angle by use of a cylindrical member rotatably mounted within the frame head tube. The steering axis is eccentric, forming an angle with respect to the axis of the cylindrical member. Rotation of the cylindrical member, with respect to the head tube, causes the steering axis to move, and thus alters the steering axis angle of inclination. The diameter of the head tube must be manufactured to be larger than normal, as the adjustment of the steering axis angle is limited by the eccentricity of the steering axis. The eccentricity is limited by the angle formed between the steering axis and the axis of the cylindrical member. This angle is, in turn, limited by the inside diameter of the head tube. It also is difficult to align the steering axis so that it remains in a vertical, longitudinal plane. Installation of such a mechanism in a motorcycle not specifically designed to accept this arrangement, is not feasible, as the eccentricity is restricted by the diameter of the cylindrical member, which has to fit within the head tube. No forward, nor vertical translation, or relocation, of the steering axis is possible.
U.S. Pat. No. 4,600,207 describes a multi piece linkage type connection mechanism between a specifically manufactured fork assembly and a specifically fabricated motorcycle frame, such that the angle of the steering axis can be adjusted, even while riding the motorcycle. The motorcycle's frame has no integral head tube, and the frame must be specifically manufactured or configured such that the mechanisms, including eccentrics and linkages, can be used. Installation on an unmodified motorcycle having a head tube defining a steering axis, is not possible.
British Patent 570439 discloses an arrangement where the fork assembly can tilt, while being constrained near its upper end by means of a plate having a slot which is oriented in a longitudinal direction. The angle of inclination of the steering axis can be adjusted by tilting the fork assembly, and then constraining the fork assembly by means of a threaded fastener passing through the elongated slot. The fork assembly and frame must be manufactured to include these various specific components. This method of altering the angle of inclination of the steering axis cannot be used with an unmodified motorcycle frame and fork assembly, and would require extensive permanent modifications to be made to any conventional motorcycle. The plate rigidly attaches to the steering fork, and does not rigidly attach to the head tube portion of the motorcycle's frame. The steering fork does not provide its steering function by pivoting, or steering, with respect to the plate. The arrangement does not relocate a steering axis to be in front of a former steering axis.
It is sometimes desirable to be able to relocate the steering axis, and thus relocate the steering assembly, of a vehicle such as a motorcycle. This cannot be accomplished without firstly designing and fabricating the motorcycle frame to provide for an adjustable steering axis, or performing extensive irreversible modifications to the motorcycle's existing frame members. What is needed, and is not found in prior art, to solve the problem of how to easily and reversibly relocate the steering axis, is a novel and convenient apparatus. Not having to cut and modify the motorcycle, and being able to subsequently revert to an original steering angle configuration, would also be desirable.